EXIF parsing fixes:
* Fix bug in parsing EXIF data that's 'SRATIONAL'. (Including Exif:Brightness)
* Add more EXIF fields we didn't support before. Also some minor reformatting.
Overhaul of IPTC/XMP parsing -- we were very broken before.
* Refactor to use PugiXML for more robust XML parsing.
* Also add a bunch of new fields from the 2010 spec, I'd been working from
  the 2008 before.
* Handle repeating IPTC tags generally, no more special case for keywords.
* Complete rewrite of XMP output, it was formatted in such a way that
  most apps weren't reading it previously.  Not almost everything makes
  it through to Lightroom and Apple Preview.App.  Still a little bit of
  work left to do, it's still ugly, but this is huge progress.
pugixmp.cpp needs an ifdef to avoid warnings on some compilers